The spokesman said the Marines were on a fact-finding mission to help the peacekeepers defend their positions at Beirut airport from artillery attacks from anti-government forces in the Shouf mountains.

Limited artillery exchanges flared around the strategic town of Souk al Gharb overlooking Beirut as United States and Saudi Arabian envoys continued trying to arrange a Lebanese cease-fire, Reuters reported.

Lebanese Army positions in the coastal town of Jubail, 20 miles north of Beirut, also came under fire for the third day, as Syrian-backed forces apparently tried to disrupt a makeshift air force base.

Meanwhile, Syria has softened its conditions for a cease-fire in Lebanon despite this week's military escalation in the central mountains and direct US Navy action on behalf of the Lebanese Army.

Officials said Saudi mediator Prince Bandar bin Sultan had succeeded in reducingthe Syrian demands, which have so far blocked an agreement between the Lebanese government and Syrian-backed forces.